1. Field of the Invention
The present invention relates to a method wherein reception of drops of water on a surface of a gas sensing element in a gas sensor is confirmed to estimate receiving conditions of the drops of water and a water confirming powder used for the method.
2. Description of Related Art
As is well known, a gas sensor is disposed in an exhaust system of an internal combustion engine of a vehicle or the like. This sensor has a gas sensing element exposed to an exhaust gas of the exhaust system and detects a concentration of a specific component (e.g., O2, NO, CO or the like) of the exhaust gas.
The gas sensing element has a plate-shaped solid electrolyte body having a high mobility for oxygen ions, a gas measurement electrode disposed on one surface of the solid electrolyte body, and a reference gas electrode disposed on the other surface of the solid electrolyte body. The measures gas electrode is exposed to the exhaust gas to be measured, and the reference gas electrode is exposed to a reference gas. The temperature of the exhaust gas reaches almost 1000° C. Further, to activate the solid electrolyte body, the solid electrolyte body is heated by a heater so as to be maintained in an activity temperature range.
During the operation of the engine, moisture included in the exhaust gas adheres to limited areas of an outer surface of the gas sensing element as drops of water, so that the temperature at the surface areas of the element receiving the drops of water is considerably lowered. Therefore, a large difference in temperature is generated between each limited area and a surface area surrounding the limited area, and thermal stress is caused in the element by the temperature difference. In this case, there is a high probability that cracks occur in the element due to this thermal stress.
To prevent the occurrence of cracks, Published Japanese Patent First Publication No. 2003-322632 discloses a testing method of tentatively receiving drops of water on a surface of a gas sensing element and confirming the reception of the drops of water to estimate receiving conditions of the drops of water. More specifically, in this testing method, carbon particles are applied to or coated on the whole surface of a gas sensing element, the element is attached to a gas sensor, and the gas sensor is disposed in an exhaust system of an internal combustion engine of a motorcar. Thereafter, the engine is tentatively operated so as to output an exhaust gas, and drops of water are attached to the surface of the element. When a drop of water is attached to a limited area of the surface of the element, carbon particles applied to the limited area are removed, and a confirmative trace of receiving the drop of water is formed on the limited area. After the operation of the engine is finished, confirmative traces of the drops of water formed on the surface of the element are observed with an operator's eye, and the reception of the drops of water is confirmed. Therefore, receiving conditions (positions, sizes and the like) of the drops of water can be estimated, and the element can be modified so as to prevent the occurrence of cracks.
However, the carbon particles are burned away in a temperature range from 400 to 500° C., while the exhaust gas output from the engine has a temperature of almost 1000° C. when the engine is actually operated for a long time to operate the car. Therefore, when it is intended to confirm the reception of drops of water in the gas sensing element under the same circumstances as those in which the element is exposed to the exhaust gas of almost 1000° C. in the actually-operated engine, there is a high probability that the carbon particles applied on the surface of the gas sensing element are burned up and disappeared. In this case, even though the gas sensing element receives drops of water, it is impossible to confirm the reception of the drops of water. That is, it is difficult to estimate receiving conditions of the drops of water under the same circumstances as those obtained in an actual operation of the engine.